Intravenous glucose tolerance test-derived glucose effectiveness in endurance-trained rats

Glucose tolerance is determined by complex interaction among several physiological processes, including insulin secretion, insulin sensitivity (S[I]), and the action of glucose itself to enhance glucose uptake and suppress its release by the liver. The combined effect of glucose to enhance glucose uptake and suppress endogenous glucose production at basal insulin has been defined as glucose effectiveness (S[G]) and is at least equal to insulin itself in the determination of glucose tolerance. The present study was undertaken to examine the effects of endurance training on S(G) in young rats. Female Sprague-Dawley rats (7 weeks old) were randomly assigned to sedentary and trained groups, and rats in the trained group had access to a running wheel for 3 weeks. Intravenous glucose tolerance tests ([IVGTTs] 500 mg/kg body weight) were performed 30 hours after the rats stopped wheel-running. The glucose disappearance constant (KG) and minimal model-derived S(I) and S(G) for trained rats were higher than for sedentary animals. Increases in S(G) were positively correlated with the relative heart weight, an index of endurance capacity. Our results confirm the previous observation in a cross-sectional study that an improved glucose tolerance in endurance-trained athletes was due to an increase in S(G) and S(I), suggesting that physical exercise is a unique physiological condition that enhances both S(G) and S(I).